Genetic Tests Used to Diagnose PWS

Because the genetics of PWS is so complicated, it usually takes more than one test to be certain whether someone has PWS and what form of it they have. The major tests that are used in the diagnosis of PWS are shown in the table. Which genetic tests are used, and in what order, will depend on a number of considerations for each individual case (see Testing Considerations).

Genetic testing requires a blood sample from the child and possibly from one or both parents as well. Families should consider genetic testing for PWS if they have:

• a baby with low muscle tone (hypotonia), poor sucking ability, and - if a boy - undescended testicles;
• a child or adult who has a number of the characteristics of Prader-Willi syndrome as listed in the Diagnostic Criteria for PWS or
• an undiagnosed child who was tested for PWS using older tests than those available today.

Testing Considerations

Which genetic tests should be done and in what order?
The approach to testing for PWS any given case will depend on a number of considerations-what tests have already been done, what expertise and laboratories are available, whether both parents are available for blood samples,and so forth. Chromosome studies are typically done in any case, but the order of the other tests-and their results will determine how many need to be done. In 1996, two national genetics groups worked together to develop guidelines on testing for Prader-Willi and Angelman syndromes. Their recommendations have been published and are available on the Internet (see References). In most cases, they recommend continued testing until the genetic cause of PWS is known.

Some testing scenarios:

If an experienced diagnostician suspects Prader-Willi Syndrome in an older child or adult who meets the Diagnostic Criteria for PWS, the FISH test might be the first test of choice because it is widely available and will detect the majority of cases of PWS. If the FISH test is positive (a deletion is found), the diagnosis of PWS is confirmed and no further testing is needed. If the FISH test comes back negative (detecting no deletion), the next step would be the DNA methylastion test. A relatively new test, DNA methylation can diagnose 99 percent of people with PWS but it does not tell whether the cause of PWS is deletion, uniparental disomy (UPD) or an imprinting mutation. If, after the negative FISH test, the methylation test confirms that the person has PWS, more testing is needed to find out whether the cause is UPD or an imprinting mutation. If the UPD test is negative in this case, the cause must be an imprinting mutaiton. At this time, imprinting mutations are diagnosed by process of elimination-positive methylation test, but negative FISH and UPD tests.

In cases where the suspicion of PWS is not as strong, or where the diagnosing physician is not as familiar with PWS, the DNA methylation test might be the best place to start. Although it is not as widely available, this test can confirm or rule out PWS at the first step. If the methylation test is positive, then additional testing can be done at the same lab to determine the specific form of PWS. Even experienced diagnosticians have sometimes midiagnosed infants as having PWS when in fact they had Angelman Syndrome. (Both syndromes can cause hypotonia in the newborn baby, and both will show a chromosome 15 deletion on the FISH test.) Starting with the methylation test avoids this problem. In cases of an imprinting mutation or other rare test findings, families may need further testing through a research laboratory, both to get an accurate diagnosis and to learn about hteir risks of having another child with PWS.

What about prenatal testing?

Prenatal testing for PWS is now available. An expectant family might wonder whether to have testing done if they have had a child with PWS previously. Although the risk of having a second baby with PWS is very low in most cases, prenatal testing can provide important reassurance to the family tha the new baby will not be affected. Counselling by a genetics professional can help a family understand their specific risks and whether testing of the fetus is important in their situation.
Prenatal testing for PWS might also be done in cases where a genetic study of the fetus (through chorionic villus sampling-CVS-or amniocentesis) shows abnormalities that raise suspicion of PWS. In one recent case, for example, a routine chromosome test done through CVS early in a woman's pregnancy found that some of the baby's cells had three chromosome 15s (called mosiac trisomy 15). This led the doctor to order a molecular test for maternal uniparental disomy (UPD) in the remaining cells. The test results showed that the baby would have PWS due to UPD.

Who should do the testing?

Families who are seeking a diagnosis or who have concerns about the risks should work with a genetics specialist who is knowledgeable about PWS and the latest in testing. The geneticist will arrange to have blood samples sent to an appropriate laboratory for testing.

Conclusion

Over the past 20 years, geneticists have unravelled many of the mysteries of PWS. Now that we can reliably test for all three genetic forms of PWS, more of our children are getting an early diagnosis, and we are discovering that there is more variation within the syndrome than once was thought. What will the next 20 years bring? Scientists surely will be able to tell us soon which specific genes are involved in PWS and, later, what those genes do. With genetic knowledge expanding at an ever-faster pace, we may in our lifetimes begin to see solutions for PWS based on new discoveries. It should be encouraging to families that many geneticists find PWS a fascinating subject for study. We wish these researchers God-speed and eagerly await findings that can change the future for our children.

References:

ASHG/ACMG Report. Diagnostic Testing for Prader-Willi and Angelman Syndromes: Report of the ASHG/ACMG Test and Technologoy Transfer Committee. American Journal of Human Genetics 58:1085-1088 (www.faseb.org/genetics/acmg/pol-22htm)

Butler, M.G. (1995) Prader-Willi Syndrome: A Guide for Parents and Professionals. Visible Ink Incorporated, New York.

Cassidy, S.B. and Schwartz, S (1998) Prader-Willi and Angelman Syndromes: Disorders of Genomic Imprinting. Medicine 77: 140-151.

Cassidy, S.B. (1995) Genetics of Prader-Willi Syndrome. In Greenswag and Alexander Management of Prader-Willi Syndrome, Second Ed. Springer-Verlag, New York.

Cassidy, S.B. (1998) Prader-Willi Syndrome. GeneClinics (www.geneclinics.org)

Mills, P.L. (1999) Laboratory Testing for Prader-Willi Syndrome. Unpublished paper. University of Chicago Genetic Services.

TEST	WHAT THIS TEST DETECTS	WHAT IT CAN'T DETECT	TEST AVAILABILITY
High Resolution Chromosomal Analysis (examination under microscope)	Large deletions and other chromosome abnormalities such as translocations and extra chromosomes	Small deletions, uniparental disomy (UPD), Imprinting mutations	Widely available
The following are classified as molecular tests:
FISH (Stands for: Fluorescence In Situ Hybridization, often done at the same time as a chromosome analysis)	Deletions of all sizes	UPD Imprinting mutations, Which parent each chromosome 15 came from (a deletion could mean either PWS or Angelman Syndrome)	Widely Available
DNA polymorphism studies (done to detect UPD, requires blood samples from both parents and child for best accuracy)	Which parent each chromosome 15 came from (if both chromosomes are from the mother, the child has PWS; if both are from the father, it's Angelman Syndrome).  Can also detect some deletions.	Imprinting mutations,  Some deletions	Not widely available
DNA methylation test (confirms or rules out PWS as a diagnosis, with over 99% accuracy)	The imprinting pattern in region 15a11-q13(normal results show both paternally and maternally inherited pattern; in PWS there is only a maternally inherited pattern, whether there is a deletion, UPD, or imprinting mutation)	Which form (molecular class) of PWS the child has: deletion, UPD, or imprinting mutation.	Not widely available